1. Field of the Invention
The invention relates to fabrication of devices formed from metallized magnetic substrates, e.g., inductors, transformers, and substrates for power applications.
2. Discussion of the Related Art
Magnetic components such as inductors and transformers are widely employed in circuits requiring energy storage and conversion, impedance matching, filtering, electromagnetic interference suppression, voltage and current transformation, and resonance. These components tend to be bulky and expensive compared to the other components of a circuit. Early manufacturing methods typically involved wrapping conductive wire around a magnetic core element or an insulating body containing magnetic core material. These early methods resulted in circuit components with tall profiles, and such profiles restricted miniaturization of the devices in which the components were used. The size restriction was particularly problematic in power circuits such as power converters.
More recent efforts to improve upon these early manufacturing methods resulted in thick film techniques and multilayer green tape techniques. In a thick film technique, a sequence of thick film screen print operations are performed using a ferrite paste and a conductor paste. Specifically, individual ferrite layers are deposited as a paste to form a substrate, while the conductor paste is deposited between the individual ferrite paste layers to form conductive patterns through the interior of the substrate. Conductor paste is also printed onto the surfaces of the resulting multilayer ferrite substrate to connect the vias, thereby forming spiral windings. Upon firing, a consolidated body containing numerous devices is typically formed.
The green tape technique uses green tape layers composed of ferrite particles and organic binder to form the substrate. Typically, as shown in FIGS. 2A to 2C, numerous holes 22 are punched through each of several green tape layers 20 (for simultaneous formation of numerous devices). As shown in FIG. 2B, the side walls of the holes 22 are subsequently coated with a conductive material 24, and then the green tape layers 20 are stacked and laminated to form a substrate 30. As shown in FIG. 2C, conductor material 32 is printed onto the opposing surfaces of the multilayer substrate 30, and connected to the conductive material 24 coated onto the side walls of the holes 22, such that continuous, conductive windings are formed. The substrate 30 is fired to form a consolidated ceramic, and, typically, a metal such as copper is electroplated onto the windings to provide improved conductivity. Such green tape techniques experience problems, however. For example, due to the numerous, relatively small vias, it is sometimes difficult to attain a uniform electroplated layer in the vias due to mass transport limitations from the electroplating bath to the via surfaces. In addition, the adhesion of the electroplated layer on the conductive material is often problematic in green tape techniques.
Improved methods for forming devices that incorporate metallized magnetic substrates, such as inductors and transformers, are desired. Particularly desired are methods that offer improved fabrication speeds and device yields from a single multilayer substrate.